Electrical test fixtures are used to test the functionality of a printed circuit board by making electrical interconnection between nodal points in the printed circuit board and a test system. The test system, or test electronics analyzer, typically comprises a computer programmed to send test signals through the test fixture to the printed circuit board to test the operability of circuit components on the printed circuit board.
A standard fixture employed for this testing is a "bed of nails" fixture. A bed of nails fixture comprises a large number of nail-like test probes having tips that make electrical contact with the nodal points of the circuit to be tested. The test probes are typically spring loaded pins inserted in receptacles that pass through and are secured relative to a supporting plate. The printed circuit is placed on top of the test probes and sealed with a gasket. A vacuum is applied through the test fixture to draw the printed circuit board down onto the spring loaded test probes to ensure good electrical contact. The vacuum is maintained until the testing is complete after which another printed circuit board is placed onto the test fixture for testing. A bed of nails type of fixture is disclosed in U.S. Pat. No. 4,352,061 issued Sep. 28, 1997 to Fairchild Camera and Instrument.
The test probes are inserted into the receptacles which extend below the lower side of the well plate. The lower end of the receptacle typically has a square wire wrap post. Other types terminal post are crimp, solder cup and round. A wire is wrapped about the receptacle post and extends in a point-to-point wiring connection to an interface connector pin inserted into a fixture interface panel. The fixture interface panel is adapted to be connected to an interface receiver of the test electronics analyzer. The point-to-point wiring of each receptacle post to a corresponding interface connector pin involves manually wire wrapping one wire or a twisted pair of wires between each of the receptacle post and interface connector pin. The extra wire in the twisted pair is connected to ground and is used to limit noise effects from the electromagnetic interference (EMI) induced by running test signals through other wires in the test fixture during the test analysis. With the miniaturization of electrical components the number of test points in a circuit has risen significantly making point-to-point wiring for each fixture a labor intensive operation.
The number of nodal contact points to be tested on the printed circuit board can range upwards to 8000 or more nodal test points. For printed circuit boards requiring in the order of 3000 to 5000 or less nodal test points it is economical practice at the time of filing the present application to use point-to-point wiring between each test probe and a corresponding contact probe. Automation of this process is not economical because each printed circuit board requires a unique design configuration. As the number of nodal points rises above 3000 to 5000 points, the wiring of the closely adjacent test pins becomes more tedious and solutions relating to the use of a wireless test fixture have been contemplated.
Wireless test fixtures solutions using rigid interface boards to interconnect the receptacle posts to the interface pin connectors of the fixture interface panel are now available. These wireless test fixtures have improved manufacturing processing times and reduced effects from EMI when compared to point-to-point wiring in the electrical test fixtures. The rigid interface board is formed with a large number of pads or openings in which the receptacle posts are inserted and soldered. In some cases, two sided probes are used to connect with the rigid interface boards. Electrically conductive traces run between the test probes and the connector terminals through this multilayer interface board. With these interface boards, the wire traces are also run to the ends of the boards and electrical connections are taken from a pin connection terminal directly to the fixture interface panel via cabling. The problem with such rigid interface boards is that the construction of the interface boards resembles that of a printed circuit board requiring capital installation costs. Since the multilayer interface board is typically a one off board for the test fixture, the price of such wireless fixtures is relatively high. Another problem occurs when a nodal test point location is changed or added during the development of the test fixture, or an engineering change is implemented. This may occur when the test fixture is being tested before shipment or later when minor engineering board changes are required. When boards are used, the addition or modification of a nodal contact point may result in a change in circuit topography requiring a new board. Also, the soldering of the test probes and connectors to the interface board is time consuming and can cause solder bridging (short circuits).
A wireless test fixture is disclosed in U.S. Pat. No. 5,157,325 where the use of rigid interface boards with test probes and interface connector pins that abuttingly contact electrical trace contact pads of the interface board. While no soldering is required, this interface board still requires circuit traces to be run on both sides of the board, and in some case on both sides of layers of a multilayer board. This board is subject to the same disadvantages noted above relating to printed circuit boards.
The introduction of a flexible printed circuit to act as the interface device in a wireless test fixture is disclosed in U.S. Pat. No. 5,252,916 issued Oct. 12, 1993 to Everett Charles Technologies Inc. The flexible circuit is attached to the well plate and the interface receiver panel by adhesive so that the flexible circuit does not move relative to these rigid substrates. This patent teaches the use of nail like test probes without springs and a diaphragm secured across a portion of the bottom layer of the flexible circuit substrate and the probe pins. The probes have mushroom shaped heads against which the diaphragm is moved when pressurized to bring the pins into good electrical contact with the nodal points of the printed circuit board. The patent is concerned with the use of a diaphragm to force the test probe pins into contact with the printed circuit board. To ensure electrical contact between the probes and flexible circuit traces, contact eyelet's are fastened to the flexible circuit over an aligned aperture in the flexible circuit. The probes extend through the eyelet's to each of the circuit traces at the interface pin connector. The fastening of thousands of eyelet's to the flexible circuit is a laborious and expensive task.
There is a need for a wireless test fixture that can be manufactured without additional component costs, in a timely manner and still have the wire integrity associated with wire wrapping.